Conventionally, in an optical pickup used for optical information recording-reproduction apparatuses, a light beam is focused onto a recording surface of an optical disk through an objective lens. The objective lens is installed in an objective-lens driving device which is controlled according to a varying amount of a detected light beam reflected from the optical disk so that the light beam is focused onto a target track on the recording surface of the optical disk, and accurately traces the target track.
The objective lens, which is installed in the objective-lens driving device, is driven in two directions: a focusing direction and a tracking direction. The focusing direction is parallel to the light axis, that is, perpendicular to the recording surface of the optical disk, and the tracking direction is orthogonal to the light axis, that is, parallel to the recording surface of the optical disk.
In such an objective-lens driving device, it is necessary to drive the objective lens without causing any tilt between the central axis of the objective lens and the light axis. For this reason, an objective-lens driving-use support body, which supports the objective lens, is provided with a parallel link mechanism that shifts the objective lens in the focusing direction while maintaining its horizontal position by means of joints (hinges) having a partially thinner portion. The support body is further furnished with a hinge section that is made flexible by providing a partially thinner portion at the rotation center thereof with respect to the objective lens, which is rocked in the tracking direction.
In adjusting the optical pickup, the objective lens is attached to the lens holder so that the objective lens is located at an optimum position with respect to the axis of the light beam. After adjusting the mechanism, it is desirable that the axis of the objective lens is kept coincident with the light axis under any environmental conditions of high temperature or low temperature. Although such an actuator, which employs the objective-lens driving-use support body made of resin, has advantages in that it can be efficiently assembled at a low cost, etc., it also has a disadvantage that the axis of the objective lens tends to deviate from the light axis due to temperature changes.
In particular, the hinge section, which is provided with a means to shift the objective lens in the focusing direction and/or the tracking direction, is easily deformed due to high temperatures. When shelf tests are conducted under high temperatures, the hinge section becomes softened and causes the optical pickup to deteriorate in its performance as is described in the following.
When an optical pickup, whose objective lens has been adjusted so as to be located at an appropriate position with respect to the light axis, is left under high temperature, the hinge section provided as a thinner portion softens. As a result, the hinge section is unable to hold the weight of the lens holder, which includes the driving coils and objective lens. This causes the lens holder to be dislocated downward and to come into contact with the base plate of the optical pickup at one portion of the bottom surface thereof. If this state is left untreated, the bottom surface of the lens holder will entirely contact the surface of the base plate.
Here, it is noted that although the positional relationship between the central axis of the objective lens and the light axis has been properly adjusted, the positional relationship between the central axis of the objective lens and the surface of the base plate is not necessarily constant due to errors in the assembling process or other processes. Therefore, in the state where the bottom surface of the lens holder has been kept in contact with the surface of the base plate, if the ambient temperature drops to normal temperature or low temperature, the lens holder is raised to its original state as the hinge section recovers its original modulus of elasticity; however, in fact, the lens holder will return to a somewhat different state from the original adjusted state seen before it underwent the high temperature shelf test. This change in position results in serious adverse effects on the optical performance; therefore, it has been desired to minimize the change of positional movement. Further, in the case where an optical information recording-reproduction apparatus is used in such a manner as to cause a tilt on the objective-lens driving device installed therein, it is hard to maintain the proper position of the objective lens with respect to the tracking direction due to the weight of the lens holder.
By adopting the conventional methods such as the fulcrum axis method using metal materials and the parallel arm method using plate springs and flexible wires, the above problems can be reduced; however, those methods arise other problems of scratches and dust associated with their sliding-axis mechanism, and are not effective in the working efficiency of the assembly.
In an objective-lens driving device taught in the Japanese Laid-Open Patent Publication No. 138536/1988 (Tokukaishou 63-138536), a mold-type resin support body wherein a tracking-use hinge is formed is adopted. Here, at least two iron pieces are symmetrically disposed in a plane that is perpendicular to a line connecting between the objective lens and the tracking-use hinge and that passes through the center of the tracking-use hinge. During non-driving state, the neutral position of the objective lens with respect to the tracking direction is achieved by attracting forces of magnets that are exerted between the corresponding iron pieces.
The operation of the above-mentioned objective-lens driving device can be theoretically fulfilled if those magnets disposed symmetrically have a completely uniform magnetic flux density and the distance between each magnet and the corresponding iron piece is completely constant. In actual practice, however, even the slightest error in the magnetic flux density or in the distance may cause a slight difference in the attracting force of the magnet that is exerted on the corresponding iron piece. As a result, the objective lens is always pulled toward either of the magnets that has a stronger attracting force. When a high-temperature test is conducted on an objective-lens driving device of this type, the error in the magnetic flux density of the magnets and the error in the distance between the magnet and the iron piece are increased to the extreme due to the softened molded-type resin support body, thereby causing the position of the objective lens to be dislocated. Further, since the frequency characteristic of the objective-lens driving device is changed due to the dislocation of the position of the tracking-use hinge, problems such as resonance may be encountered. Therefore, it is doubtful whether the above-mentioned objective-lens driving device can be put into practical use.
On the other hand, in an objective-lens driving device taught in the Japanese Laid-Open Patent Publication No. 226031/1985 (Tokukaishou 60-226031), the neutral position of the objective lens is achieved by two plate springs that are aligned opposite to each other in the focusing direction. Here, an attracting force, which is exerted between a magnet secured to the rocking end of the objective lens holder and a magnetic material aligned face to face with the magnet, is used to assist in the dislocation of the plate springs in the focusing direction.
However, even a slightest twisting force applied onto the plate springs may be amplified by the attracting force between the magnet and the magnetic material; hence, it is doubtful whether this objective-lens driving device can be put into practical use.
As described above, in the conventional objective-lens driving devices, the objective-lens driving-use support body including such a mechanism as the parallel link mechanism that is made of resin has advantages in working efficiency of assembly and low cost; however, it may cause adverse effects on the optical performance of the optical pickup because the parallel link mechanism or other mechanism may be deformed due to high temperatures.
Moreover, in the objective-lens driving device which suffers from lack of balance in the tracking direction around the tracking-use hinge as its center, if the optical information recording-reproduction apparatus is used with its optical pickup being tilted, especially in the case where it is tilted in such a manner that the tracking direction coincides with the vertical direction, the objective lens is not maintained in the neutral position due to the weight of the lens holder.
Furthermore, in the structure wherein two pairs of the magnet and the iron pieces are disposed at respective places, which are symmetrical with the tracking-use hinge as the center, in order to prevent the lens holder from being dislocated downward, tilt of the objective lens due to fluctuations of the attracting force presents a problem to be solved.